The present invention relates to the field of transmission lines forming part of a monolithic integrated circuit chip and, more particularly, to such chips having an epitaxial layer formed thereon.
Transmission lines formed by metallized strips deposited on the passivated surface of a substrate are well known, and are used for many purposes beyond mere connection of one point on an IC chip with another point. Such lines may operate in one of three general modes. When the product of the frequency and the resistivity of the substrate is large enough to produce a small dielectric loss angle, the substrate acts as a dielectric, and the line operates in a mode closely resembling the TEM mode, which may be termed the dielectric "quasi-TEM" mode. The dielectric loss in the SiO.sub.2 layer can be ignored and almost all of the energy is transmitted through the silicon layer at nearly the velocity of light in a vacuum.
When the product of the frequency and the conductivity (1/R) is large, the substrate appears to be a lossy conductor wall. With a very thin SiO.sub.2 layer, the dispersion effect is controlled by skin effect in the substrate and the device operates in a "skin effect" mode.
However, when the frequency is not as high and the resistivity is in the moderate range; e.g., 10.sup.-4 to 10.sup.+2 ohms-cm, the propagation velocity may be slowed down to a few hundredths of the velocity of light in a vacuum. This mode of operation has been termed the "slow wave" mode. A complete description of the theoretical basis of the three modes of operation may be found in "Properties of Microstrip Line on Si-SiO.sub.2 System" published in the IEEE "Transactions on Microwave Theory and Techniques", Vol. MTT-19, No. 11, November 1971, pp. 869-881.
The development of the epitaxial process provided many advantages for integrated circuit designs and many monolithic integrated circuits are presently made with an epitaxial layer formed on the substrate. To provide a slow-wave transmission line has heretofore required a Schottky metal which was not compatible with bipolar processes. It is highly desirable to be able to provide a slow wave transmission line by using the normal process steps in epitaxial integrated circuit fabrication.
Another slow wave transmission line, filed as of even date with the present application, and bearing U.S. Ser. No. 944,059, is also compatible with bipolar integrated circuit construction. This other transmission line has no buried layer, but has a p+ diffusion beneath the transmission line. It is, therefore, not voltage controllable as is the present invention, but has better slow wave performance.